Structure and formation method of semiconductor device structure

ABSTRACT

Structures and formation methods of a semiconductor device structure are provided. The semiconductor device structure includes a gate stack over a semiconductor substrate and a cap element over the gate stack. The cap element has an upper portion and a lower portion, and the upper portion is wider than the lower portion. The semiconductor device structure also includes a spacer element over a sidewall of the cap element and a sidewall of the gate stack.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims the benefit of U.S. Provisional Application No. 62/165,369, filed on May 22, 2015, the entirety of which is incorporated by reference herein.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapid growth. Technological advances in IC materials and design have produced generations of ICs. Each generation has smaller and more complex circuits than the previous generation.

In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometric size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling-down process generally provides benefits by increasing production efficiency and lowering associated costs.

However, these advances have increased the complexity of processing and manufacturing ICs. Since feature sizes continue to decrease, fabrication processes continue to become more difficult to perform. Therefore, it is a challenge to form reliable semiconductor devices at smaller and smaller sizes.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It should be noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1A-1G are cross-sectional views of various stages of a process for forming a semiconductor device structure, in accordance with some embodiments.

FIG. 2 is a cross-sectional view of a semiconductor device structure, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

Some embodiments of the disclosure are described. FIGS. 1A-1G are cross-sectional views of various stages of a process for forming a semiconductor device structure, in accordance with some embodiments. Additional operations can be provided before, during, and/or after the stages described in FIGS. 1A-1G. Some of the stages that are described can be replaced or eliminated for different embodiments. Additional features can be added to the semiconductor device structure. Some of the features described below can be replaced or eliminated for different embodiments.

As shown in FIG. 1A, a semiconductor substrate 100 is provided. In some embodiments, the semiconductor substrate 100 is a bulk semiconductor substrate, such as a semiconductor wafer. For example, the semiconductor substrate 100 is a silicon wafer. The semiconductor substrate 100 may include silicon or another elementary semiconductor material such as germanium. In some other embodiments, the semiconductor substrate 100 includes a compound semiconductor. The compound semiconductor may include gallium arsenide, silicon carbide, indium arsenide, indium phosphide, another suitable material, or a combination thereof.

In some embodiments, the semiconductor substrate 100 includes a semiconductor-on-insulator (SOI) substrate. The SOI substrate may be fabricated using a separation by implantation of oxygen (SIMOX) process, a wafer bonding process, another applicable method, or a combination thereof.

In some embodiments, one or multiple fin structures are formed. As shown in FIG. 1A, one of the fin structures (the fin structure 101) is shown. In some embodiments, multiple recesses (or trenches) (not shown) are formed in the semiconductor substrate 100. As a result, multiple fin structures including a fin structure 101 are formed between the recesses. In some embodiments, one or more photolithography and etching processes are used to form the recesses.

As shown in FIG. 1A, isolation features (not shown) are formed in the recesses to surround a lower portion of the fin structure 101, in accordance with some embodiments. The isolation features are used to define and electrically isolate various device elements formed in and/or over the semiconductor substrate 100. In some embodiments, the isolation features include shallow trench isolation (STI) features, local oxidation of silicon (LOCOS) features, other suitable isolation features, or a combination thereof.

In some embodiments, each of the isolation features has a multi-layer structure. In some embodiments, the isolation features are made of a dielectric material. The dielectric material may include silicon oxide, silicon nitride, silicon oxynitride, fluoride-doped silicate glass (FSG), low-K dielectric material, another suitable material, or a combination thereof. In some embodiments, an STI liner (not shown) is formed to reduce crystalline defects at the interface between the semiconductor substrate 100 and the isolation features. Similarly, the STI liner may also be used to reduce crystalline defects at the interface between the fin structures and the isolation features.

In some embodiments, a dielectric material layer is deposited over the semiconductor substrate 100. The dielectric material layer covers the fin structures including the fin structure 101 and fills the recesses between the fin structures. In some embodiments, the dielectric material layer is deposited using a chemical vapor deposition (CVD) process, a spin-on process, another applicable process, or a combination thereof. In some embodiments, a planarization process is performed to thin down the dielectric material layer until the fin structure 101 is exposed. The planarization process may include a chemical mechanical polishing (CMP) process, a grinding process, an etching process, another applicable process, or a combination thereof. Afterwards, the dielectric material layer is etched back to below the top of the fin structure 101. As a result, the isolation features are formed. The fin structures including the fin structure 101 protrude from the isolation features, in accordance with some embodiments.

As shown in FIG. 1A, a gate dielectric layer 104 is deposited over the isolation features and the fin structure 101, in accordance with some embodiments. In some embodiments, the gate dielectric layer 104 is made of silicon oxide, silicon nitride, silicon oxynitride, dielectric material with high dielectric constant (high-K), another suitable dielectric material, or a combination thereof. Examples of high-K dielectric materials include hafnium oxide, zirconium oxide, aluminum oxide, hafnium dioxide-alumina alloy, hafnium silicon oxide, hafnium silicon oxynitride, hafnium tantalum oxide, hafnium titanium oxide, hafnium zirconium oxide, another suitable high-K material, or a combination thereof. In some embodiments, the gate dielectric layer 104 is a dummy gate dielectric layer which will subsequently be removed. In some other embodiments, the gate dielectric layer 104 is not formed.

In some embodiments, the gate dielectric layer 104 is deposited using a chemical vapor deposition (CVD) process, an atomic layer deposition (ALD) process, a thermal oxidation process, a physical vapor deposition (PVD) process, another applicable process, or a combination thereof.

Afterwards, a gate electrode 106 is formed over the gate dielectric layer 104 to cover a portion of the fin structure 101, as shown in FIG. 1A in accordance with some embodiments. In some embodiments, the gate electrode 106 is a dummy gate electrode which will be replaced with a metal gate electrode. In some embodiments, the gate electrode 106 is made of polysilicon. In some embodiments, the portion of the fin structure 101 under the gate electrode 101 serves as a channel region of a transistor that will be formed.

In some embodiments, a gate electrode layer is deposited over the gate dielectric layer 104 using a CVD process or another applicable process. In some embodiments, the gate electrode layer is made of polysilicon. Afterwards, a patterned hard mask layer (not shown) is formed over the gate electrode layer, in accordance with some embodiments. The patterned hard mask layer may used to pattern the gate electrode layer into one or more gate electrodes including the gate electrode 106 shown in FIG. 1A. In some embodiments, the gate dielectric layer 104 is also patterned, as shown in FIG. 1A. The gate electrode 106 and the gate dielectric layer 104 may together form a gate stack 109.

In some embodiments, the gate stack 109 is a dummy gate stack and will be replaced with a metal gate stack. In some embodiments, the gate stack 109 surrounds side surfaces and a top surface of the fin structure 101 and further extends over the semiconductor substrate 100.

In some embodiments, the patterned hard mask layer includes a first hard mask layer and a second hard mask layer. The first hard mask layer is between the gate electrode layer and the second hard mask layer. In some embodiments, the first hard mask layer is made of silicon nitride. In some embodiments, the second hard mask layer is made of silicon oxide. In some embodiments, the second hard mask layer is thicker than the first mask layer.

In some embodiments, sealing elements (not shown) are formed over sidewalls of the gate stack 109. The sealing elements may be used to protect the gate stack 109 and/or assist in a subsequent process for forming lightly-doped source/drain (LDS/D) regions. In some embodiments, an ion implantation process is used to form the LDS/D regions. In some other embodiments, the sealing elements are not formed. In some other embodiments, the LDS/D regions are not formed.

Afterwards, spacer elements 108 are formed over sidewalls of the gate stack 109, as shown in FIG. 1A in accordance with some embodiments. The spacer elements 108 may be used to protect the gate stack 109 and/or assist in a subsequent process for forming source/drain features. In some embodiments, the spacer elements 108 are made of a dielectric material. The dielectric material may include silicon nitride, silicon oxynitride, silicon oxide, another suitable material, or a combination thereof.

In some embodiments, a dielectric material layer is deposited over the semiconductor substrate 100 and the gate stack 109. The dielectric material layer may be deposited using a CVD process, an ALD process, a spin-on process, another applicable process, or a combination thereof. Afterwards, the dielectric material layer is partially removed using an etching process, such as an anisotropic etching process. As a result, the remaining portions of the dielectric material layer over the sidewalls of the gate stack 109 form the spacer elements 108.

As shown in FIG. 1A, source/drain features 112 are formed over the portions of the fin structure 101 near the gate stack 109, in accordance with some embodiments. In some embodiments, the fin structure 101 is partially removed to form recesses near the spacer elements 108. Afterwards, an epitaxial growth process is performed to form the source/drain features 112, as shown in FIG. 1A in accordance with some embodiments. In some embodiments, the source/drain features 112 are also used as stressors that can apply strain or stress on the channel region between the source/drain features 112. The carrier mobility may be improved accordingly.

As shown in FIG. 1A, a dielectric layer 114 is formed to surround the gate stack 109, in accordance with some embodiments. In some embodiments, a dielectric material layer is deposited to cover the source/drain features 112, the spacer elements 108, and the gate stack 109. Afterwards, a planarization process is used to partially remove the dielectric material layer. The dielectric material layer may be partially removed until the gate electrode 106 is exposed. As a result, the dielectric layer 114 is formed.

In some embodiments, the dielectric material layer is made of silicon oxide, silicon oxynitride, borosilicate glass (BSG), phosphoric silicate glass (PSG), borophosphosilicate glass (BPSG), fluorinated silicate glass (FSG), low-k material, porous dielectric material, another suitable material, or a combination thereof. In some embodiments, the dielectric material layer is deposited using a CVD process, an ALD process, a spin-on process, another applicable process, or a combination thereof. In some embodiments, the planarization process includes a CMP process, a grinding process, an etching process, another applicable process, or a combination thereof.

Afterwards, multiple etching operations are performed to remove the gate electrode 106, in accordance with some embodiments. In some embodiments, these etching operations are performed within a same process chamber.

As shown in FIG. 1B, a portion of the gate electrode 106 is removed to form a recess 116 a in a first etching operation, in accordance with some embodiments. The recess 116 a is surrounded by the spacer elements 108 and the gate electrode 106 remained between the spacer elements 108. In some embodiments, the recess 116 a has a depth that is in a range from about 10 nm to about 20 nm. In some cases, the spacer elements 108 may slightly collapse towards the recess 116 a after the partial removal of the gate electrode 106. As a result, the opening of the recess 116 a becomes smaller.

In some embodiments, the first etching operation involves plasma. In some embodiments, the reaction gas used in the first etching operation includes HBr, Cl₂, another similar gas, another suitable gas, or a combination thereof.

As shown in FIG. 1C, a portion of the gate electrode 106 is further removed and the spacer elements 108 are also laterally etched in a second etching operation, in accordance with some embodiments. In some embodiments, an etching rate of the first etching operation to the spacer elements 108 is lower than an etching rate of the second etching operation to the spacer elements 108.

After the second etching operation, the recess (116 a) becomes deeper and is denoted as 116 b. In some embodiments, the recess 116 b has a depth that is in a range from about 20 nm to about 35 nm. In some embodiments, the recess 116 b becomes wider along a direction from a bottom of the recess 116 b towards a top of the recess 116 b, as shown in FIG. 1C.

In some embodiments, the condition of the second etching operation is fine-tuned to laterally etch upper portions of the spacer elements 108, as shown in FIG. 1C. In some embodiments, the second etching operation involves plasma. In some embodiments, the reaction gas used in the second etching operation includes CHF₃, HBr, O₂, Ar, He, another similar gas, another suitable gas, or a combination thereof. In some embodiments, a volumetric concentration of CHF₃ is in a range from about 50% to about 90%. In some embodiments, a volumetric concentration of HBr is in a range from about 20% to about 30%. In some embodiments, a volumetric concentration of O₂ is in a range from about 10% to about 40%.

Due to the laterally etching of the spacer elements 108, the recess 116 b has a larger opening. In some embodiments, the recess 116 b has a slanted sidewall. In some other embodiments, the recess 116 b has curved sidewalls. In some embodiments, widths of the recess 116 b gradually increase along a direction from a bottom of the recess 116 b towards the top of the recess 116 b. In some embodiments, the recess 116 b gradually becomes larger along a direction from a top of the gate electrode 106 towards the top of the recess 116 b.

As shown in FIG. 1D, the remaining portion of the gate electrode 106 is further removed in a third etching operation, in accordance with some embodiments. In some embodiments, an etching rate of the second etching operation to the spacer elements 108 is higher than an etching rate of the third etching operation to the spacer elements 108. In some embodiments, the spacer elements 108 are substantially not removed during the third etching operation.

After the third etching operation, the recess (116 b) becomes deeper and denoted as 116 c. In some embodiments, the recess 116 c has a depth that is in a range from about 60 nm to about 85 nm. Afterwards, the gate dielectric layer 104 is removed, in accordance with some embodiments. The recess 116 c exposes the fin structure 101, as shown in FIG. 1D.

In some embodiments, the third etching operation involves plasma. In some embodiments, the reaction gas used in the second etching operation includes HBr, Cl₂, another similar gas, another suitable gas, or a combination thereof. In some other embodiments, one or more other etching operations are performed before, during, between, and/or after the first, the second, and the third etching operations.

As shown in FIG. 1E, metal gate stack layers are deposited to fill the recess 116 c (or trench) between the spacer elements 108, in accordance with some embodiments. The metal gate stack layers may include a gate dielectric layer 118, a work function layer 120, and a conductive filling layer 122. One or more other layers may be formed between the metal gate stack layers. For example, a barrier layer is formed between the gate dielectric layer 118 and the work function layer 120. A blocking layer may be formed between the work function layer 120 and the conductive filling layer 122. In some embodiments, the filling of the metal gate stack layers becomes easier since the recess 116 c is widened (compared with the recess 116 a shown in FIG. 1B) after the etching operations mentioned above.

In some embodiments, the gate dielectric layer 118 is made of a dielectric material with high dielectric constant (high-K). The gate dielectric layer 118 may be made of hafnium oxide, zirconium oxide, aluminum oxide, hafnium dioxide-alumina alloy, hafnium silicon oxide, hafnium silicon oxynitride, hafnium tantalum oxide, hafnium titanium oxide, hafnium zirconium oxide, another suitable high-K material, or a combination thereof.

The work function layer 120 is used to provide the desired work function for transistors to enhance device performance, such as improved threshold voltage. In some embodiments, the work function layer 120 is an n-type metal layer capable of providing a work function value suitable for the device, such as equal to or less than about 4.5 eV. In some embodiments, the work function layer 120 is a p-type metal layer capable of providing a work function value suitable for the device, such as equal to or greater than about 4.8 eV.

The n-type metal layer may include metal, metal carbide, metal nitride, or a combination thereof. For example, the n-type metal layer includes titanium nitride, tantalum, tantalum nitride, other suitable materials, or a combination thereof. The p-type metal layer may include metal, metal carbide, metal nitride, other suitable materials, or a combination thereof. For example, the p-type metal includes tantalum nitride, tungsten nitride, titanium, titanium nitride, other suitable materials, or a combination thereof.

The work function layer 120 may also be made of hafnium, zirconium, titanium, tantalum, aluminum, metal carbides (e.g., hafnium carbide, zirconium carbide, titanium carbide, aluminum carbide), aluminides, ruthenium, palladium, platinum, cobalt, nickel, conductive metal oxides, or a combination thereof. The thickness and/or the compositions of the work function layer 120 may be fine-tuned to adjust the work function level. For example, a titanium nitride layer may be used as a p-type metal layer or an n-type metal layer, depending on the thickness and/or the compositions of the titanium nitride layer.

In some embodiments, the conductive filling layer 122 is made of a metal material. The metal material may include tungsten, aluminum, copper, another suitable material, or a combination thereof. The formation of the metal gate stack layers may involve multiple deposition processes. The deposition processes may include a CVD process, an ALD process, a PVD process, an electroplating process, an electroless plating process, a spin-on process, another applicable process, or a combination thereof.

As shown in FIG. 1F, a planarization process is performed to remove the portions of the metal gate stack layers outside of the recess (or trench) between the spacer elements 108, in accordance with some embodiments. As a result, a metal gate stack 123 is formed. The metal gate stack 123 includes the gate dielectric layer 118, the work function layer 120, and a gate electrode 122′ (such as a metal gate electrode) that is a portion of the conductive filling layer 122.

As shown in FIG. 1G, the metal gate stack 123 is etched back, in accordance with some embodiments. In some embodiments, the gate electrode 122′ protrudes from top surfaces of the work function layer 120 and/or the gate dielectric layer 118 after the etching back process.

In some embodiments, because the spacer elements 108 are laterally etched to provide a recess having a larger opening. Therefore, the etching process for etching back the metal gate stack 123 may be performed well due to the larger opening. The subsequent formation of a cap element and a subsequent process for forming a conductive contact to the metal gate stack 123 are significantly improved.

Afterwards, a cap element 124 is formed over the metal gate stack 123 that is etched back, as shown in FIG. 1G in accordance with some embodiments. In some embodiments, the cap element 124 is in direct contact with the metal gate stack 123. In some embodiments, the cap element 124 is in direct contact with the gate electrode 122′. In some embodiments, the cap element 124 is in direct contact with the work function layer 120. In some embodiments, the cap element 124 is in direct contact with the gate dielectric layer 118. In some embodiments, the cap element 124 is in direct contact with the spacer elements 108. In some embodiments, the gate electrode 122′ penetrates into the cap element 124. In some embodiments, the cap element 124 surrounds an upper portion of the gate electrode 122′. In some embodiments, the cap element 124 continuously surrounds an upper portion of the gate electrode 122′.

In some embodiments, the cap element 124 is made of a dielectric material. The dielectric material may include silicon nitride, silicon oxynitride, another suitable material, or a combination thereof. In some embodiments, a protection material layer (such as a dielectric layer) is deposited over the dielectric layer 114, the spacer elements 108, and the metal gate stack 123 that is etched back. In some embodiments, the protection material layer is made of a dielectric material. The dielectric material may include silicon nitride, silicon oxynitride, silicon carbide, silicon carbon nitride, oxide, another similar material, another suitable material, or a combination thereof. In some embodiments, the protection material layer is deposited using a CVD process, an ALD process, a spin-on process, another applicable process, or a combination thereof.

Afterwards, the portion of the protection material layer outside of the recess between the spacer elements 108 is removed, in accordance with some embodiments. As a result, the remaining portion of the protection material layer forms the cap element 124, as shown in FIG. 1G. In some embodiments, a planarization process is used to partially remove the protection material layer to achieve the formation of the cap element 124. In some embodiments, the planarization process includes a chemical mechanical polishing (CMP) process, a grinding process, an etching process, another applicable process, or a combination thereof.

As shown in FIG. 1G, the cap element 124 has a top 124 t and a bottom 124 b. The cap element 124 has a first width W₁ near the metal gate stack 123 (or the bottom 124 b) and a second width W₂ near the top 124 t of the cap element 124. The width W₂ is greater than the width W₁. In some embodiments, the first width W₁ is in a range from about 25 nm to about 35 nm. In some embodiments, the second width W₂ is in a range from about 27 nm to about 37 nm. In some embodiments, the cap element 124 has an upper portion and a lower portion. The upper portion is wider than the lower portion, as shown in FIG. 1G.

In some embodiments, the cap element 124 gradually becomes narrower along a direction from the top 124 t towards the bottom 124 b of the cap element 124. In some embodiments, the cap element 124 gradually becomes narrower along a direction from the top 124 t towards the metal gate stack 123. In some embodiments, the spacer element 108 gradually becomes narrower along a direction from the bottom 124 b of the cap element 124 towards a top 108 t of the spacer element 108.

As shown in FIG. 1G, there is an angle α between a bottom 125B and a sidewall 125V of the metal gate stack 123. In some embodiments, the angle α should be carefully controlled to be within a suitable range. In some embodiments, the angle α is in a range from about 85 degrees to about 95 degrees. As shown in FIG. 1G, there is an angle θ between a sidewall 125S of the cap element 124 and an imaginary plane L extending from the sidewall 124V of the metal gate stack 123 towards the top 124 t of the cap element 124. In some embodiments, the angle θ should be carefully controlled to be within a suitable range. In some embodiments, the angle θ is in a range from about 1 degree to about 10 degrees.

Many variations and/or modifications can be made to embodiments of the disclosure. FIG. 2 is a cross-sectional view of a semiconductor device structure, in accordance with some embodiments. Like reference numbers are used to designate like elements. In some embodiments, the source/drain features are doped regions formed within the fin structure 101. In some other embodiments, the fin structure 101 is not formed. In these cases, the source/drain features are doped regions formed in the semiconductor substrate 100.

Embodiments of the disclosure form a semiconductor device structure having a gate stack with spacer elements over sidewalls of the gate stack. Upper portions of the spacer elements and the gate stack are partially removed to form a recess over the gate stack and between the spacer elements. The recess has a wider upper portion than a lower portion of the recess. Afterwards, a remaining portion of the gate stack is removed. Afterwards, a metal gate stack is formed in the recess and then etched back. A cap element is then formed over the metal gate stack and between the spacer elements. Due to the profile of the recess, the qualities of the processes of the removal of the remaining portion of the gate stack, the formation of the metal gate stack, the etching back of the metal gate stack, and the formation of the cap element are improved.

In accordance with some embodiments, a semiconductor device structure is provided. The semiconductor device structure includes a gate stack over a semiconductor substrate and a cap element over the gate stack. The cap element has an upper portion and a lower portion, and the upper portion is wider than the lower portion. The semiconductor device structure also includes a spacer element over a sidewall of the cap element and a sidewall of the gate stack.

In accordance with some embodiments, a semiconductor device structure is provided. The semiconductor device structure includes a gate stack over a semiconductor substrate. The semiconductor device structure also includes a cap element over the gate stack, and the cap element has a first width near the gate stack and a second width near an upper portion of the cap element. The second width is greater than the first width. The semiconductor device structure further includes a spacer element over a sidewall of the cap element and a sidewall of the gate stack.

In accordance with some embodiments, a method for forming a semiconductor device structure is provided. The method includes forming a gate electrode over a semiconductor substrate and forming spacer elements over sidewalls of the gate electrode. The method also includes partially removing the gate electrode and the spacer elements to form a recess between the spacer elements. The recess becomes wider along a direction from a bottom of the recess towards a top of the recess. The method further includes removing a remaining portion of the gate electrode such that the recess becomes deeper. In addition, the method includes forming a metal gate stack in the recess after the remaining portion of the gate electrode is removed. The method also includes forming a cap element above the metal gate stack and between the spacer elements.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

1. A semiconductor device structure, comprising: a gate stack over a semiconductor substrate; a cap element over the gate stack, wherein the cap element has an upper portion and a lower portion, and the upper portion is wider than the lower portion; and a spacer element over a sidewall of the cap element and a sidewall of the gate stack.
 2. The semiconductor device structure as claimed in claim 1, wherein the gate stack comprises a work function layer and a gate electrode surrounded by the work function layer.
 3. The semiconductor device structure as claimed in claim 2, wherein the cap element is in direct contact with the work function layer or the gate electrode.
 4. The semiconductor device structure as claimed in claim 2, wherein the gate electrode protrudes from a top surface of the work function layer.
 5. The semiconductor device structure as claimed in claim 4, wherein the gate electrode penetrates into the cap element.
 6. The semiconductor device structure as claimed in claim 1, wherein the cap element gradually becomes narrower along a direction from a top of the cap element towards the gate stack.
 7. The semiconductor device structure as claimed in claim 1, wherein the spacer element gradually becomes narrower along a direction from a bottom of the cap element towards a top of the spacer element.
 8. The semiconductor device structure as claimed in claim 1, wherein the cap element is in direct contact with the spacer element.
 9. The semiconductor device structure as claimed in claim 1, wherein the gate stack comprises a gate dielectric layer, and the gate dielectric layer is in direct contact with the cap element.
 10. The semiconductor device structure as claimed in claim 1, wherein there is an angle between an upper portion of the sidewall of the cap element and an imaginary plane extending from the sidewall of the gate stack towards a top of the cap element, and the angle is in a range from about 1 degree to about 10 degrees.
 11. A semiconductor device structure, comprising: a gate stack over a semiconductor substrate; a cap element over the gate stack, wherein the cap element has a first width near the gate stack and a second width near an upper portion of the cap element, and the second width is greater than the first width; and a spacer element over a sidewall of the cap element and a sidewall of the gate stack.
 12. The semiconductor device structure as claimed in claim 11, wherein the cap element is made of a dielectric material.
 13. The semiconductor device structure as claimed in claim 11, wherein the cap element is in direct contact with the gate stack and the spacer element.
 14. The semiconductor device structure as claimed in claim 11, wherein the gate stack comprises a work function layer and a gate electrode surrounded by the work function layer.
 15. The semiconductor device structure as claimed in claim 14, wherein the cap element surrounds an upper portion of the gate electrode. 16.-20. (canceled)
 21. A semiconductor device structure, comprising: a gate stack over a semiconductor substrate; a cap element over the gate stack, wherein the cap element has a lower portion and an upper portion, the lower portion is substantially as wide as the gate stack, and the upper portion is wider than the lower portion; and a spacer element over a side surface of the cap element and a sidewall of the gate stack.
 22. The semiconductor device structure as claimed in claim 21, wherein the upper portion of the cap element gradually becomes narrower along a direction from a top of the cap element towards the lower portion of the cap element.
 23. The semiconductor device structure as claimed in claim 21, wherein the lower portion of the cap element has a sidewall substantially aligned with the sidewall of the gate stack.
 24. The semiconductor device structure as claimed in claim 21, further comprising a dielectric layer over the semiconductor substrate and surrounding the gate stack and the spacer element, wherein a top surface of the dielectric layer is substantially coplanar with a top of the cap element.
 25. The semiconductor device structure as claimed in claim 21, wherein the cap element is made of a dielectric material. 